The present invention relates to a fuel cell system adapted to be supplied with fuel and compressed air at a variable flow rate.
In a fuel cell system of the related art, fuel gas (hydrogen gas) is supplied to a fuel electrode and air is supplied to an oxidation electrode, with fuel gas and air electrochemically reacting with one another to generate electric power output.
In such a fuel cell system, it has been a usual practice to have a fuel cell to be supplied with compressed air delivered from a compressor. In order to drive the compressor with less energy consumption and to optimize the operation of the fuel cell throughout various operating conditions thereof, in Japanese Patent Application Laid-Open Publication No. 7-14599, the fuel cell system includes a compressor connected to a common shaft. The shaft is connected to a start-up motor and a main motor. The start-up motor is used for variably drive the compressor and is specifically suited for use in an automotive field. This start-up motor is supplied with electric power from a vehicular battery, whereas the main motor is supplied with electric power directly from a fuel cell via an electric current controller. During start-up of the fuel cell system, the compressor is driven with the start-up motor until a minimum air volume flow rate Vmin and a minimum operating pressure Pmin are enabled to be utilized. During such a time interval, the main motor is de-energized. In this instance, the minimum air volume flow rate Vmin and the minimum operating pressure Pmin are selected to have respective values such that the fuel cell generates electric energy at a level sufficient for the adequate operation of the whole fuel cell system.